1. Field of the Invention
The present invention relates to toners for developing electrostatic latent images. The present invention more specifically relates to a toner for developing electrostatic latent images which has compatibility between low-temperature fixing characteristics and stability.
2. Description of Related Art
Techniques recently developed in the field of electrophotographic imaging apparatuses, i.e., those for digital image processing and production of toners having smaller diameters enable formation of images with high resolution, and can provide prints produced directly from toner images in the printing field involving printing with printing plates.
These techniques can eliminate the step of making printing plates essential for the printing process. This results in quick production of high-quality prints, leading to application of these techniques especially to a small-lot printing field in which orders are received mainly in small quantities, for example, from about several hundreds to several thousands of prints.
Recent environmental concerns lead to investigation of a reduction in power consumption of imaging apparatuses, especially techniques of reducing the power consumption of fixing units. One of the techniques of reducing energy consumption of the fixing units is a so-called low-temperature fixing technique of fusing a toner at a low heating temperature, which has been examined for a reduction in warm-up time from a stand-by state and thus a reduction in energy consumption of the fixing units.
In addition to the low-temperature fixing technique, the small-lot printing field requires stable continuous printing of about several hundreds to several thousands of prints without a fluctuation in gloss level between the resulting prints.
For example, JP2011-170229A discloses use of a binder resin comprising a resin A containing a copolymer prepared with a styrene monomer and a (meth)acrylic monomer and a resin B containing a copolymer prepared with a methacrylate ester monomer and a radical polymerizable monomer having a plurality of carboxy groups.
The resin B present in an immiscible state in the resin A phase absorbs excess heat at high temperatures at which excess heat is readily applied to the fused toner. Hence, the fluidity of the toner during the low-temperature fixing process is maintained and a fluctuation in gloss is prevented.
Unfortunately, if two immiscible amorphous resins are present in the resin A phase, sufficient sharp-melt characteristics cannot be ensured, and fixing at significantly low temperature cannot be achieved.
To satisfy a requirement on a further reduction in temperature of the low-temperature fixing process, JP2013-225096A and JP2014-167602A each disclose a toner comprising a crystalline resin contained in an amorphous polyester resin to reduce the viscoelasticity of the toner being fused.
Unfortunately, the fused toner has a low storage modulus and a low loss modulus and the viscoelasticity of the toner is specified only at a certain temperature. Hence, a subtle change in temperature near the fixing temperature will lead to a significant variation in viscoelasticity, resulting in hot offsetting.